Fig. 1. High ACE2 and TMPRSS2 expression in lung epithelial cells from COVID-19 patients. (A) The t-SNE plot displayed the major cell types (epithelial, immune and others) in 23 clusters for bronchoalveolar lavage fluid (BALF) samples from 6 severe (S) and 3 recovered mild COVID-19 patients (M), as well as 10 healthy controls (HC). (B) The t-SNE plot displayed RNA expression of ACE2 or TMPRSS2. Right panel shows ACE2 or TMPRSS2 expression in lung epithelial cells from different groups using violin plot. (C) Dot plot of ACE2 orTMPRSS2 expression for each cell-type of lung epithelial cells from different groups. Dot size represents the percentage of cells in individual clusters expressing a given gene. (D)The pie chart shows the percentages of ACE2- o r TMPRSS2- positive cells in club (cluster 14) and ciliated (cluster 15) cells. (E) Violin plot of expression values of ACE2 or TMPRSS2 in different cell types of lung epithelial cells from different group. (F) The top 5 enriched signaling pathways of up-regulated (red) or down-regulated (green) genes in lung epithelial cells after SARS-CoV-2 infection (severe vs. health). * P < 0.05; ** P < 0.01; *** P < 0.001.
Fig. 2. SARS-CoV-2 infection induced imbalanced host immune response in severe COVID-19 patients. (A) Dot plot of ACE2 or TMPRSS2 expression for each cell-type of lung immune cells from different groups. Dot size represents the percentage of cells in individual clusters expressing a given gene. (B) Top 5 enriched signaling pathways of markers genes for macrophage (cluster 2, 3 and 6). (C) The percentages of different immune cell types of all CD45+ cells in BALF of severe (S), recovered mild COVID-19 patients (M), and healthy controls (HC). (D) Heatmaps of transcript level of candidate genes in different immune cell types. * P < 0.05; ** P < 0.01; *** P < 0.001.
Fig.3. SARS-CoV-2 infection resulted in abnormal epithelial-immune cell interaction in lung. (A-C) We evaluated intercellular communications based on expression of ligand receptor pAIrs among different cell types by CellPhoneDB. Club-immune cells (A), ciliated-immune cells (B), and immune-immune communications (C) in severe (S, n=6),recovered mild (M, n=3) COVID-19 patients, and healthy controls (HC, n=10) were shown. Only significant interactions were calculated, and the number of interactions was depicted in violin plot, or normalized against those of healthy controls in heatmap. (C) Heat map depicting different ligand receptor interactions among different cell types. Interaction strengths are color coded. [One-way Analysis of Variance (ANOVA) followed by Tukey multiple comparisons (A-C)]. * P < 0.05; ** P < 0.01; *** P < 0.001; **** P < 0.0001.
Fig. 4. SARS-CoV-2 infection and host immune response in COVID-19 patients. In COVID-19 patients, the SARS-CoV-2 may infect ciliated cells, club cells, and basal cells expressing ACE2 and TMPRSS2 in lung epithelium and actively replicate in host cells. This could lead to activation of pro-inflammatory signaling and production of pro-inflammatory cytokines which subsequently attract both innate and adaptive immune cells including neutrophils, macrophages and T cells to the infection site to fight virus and virus-infected cells. Besides, the immune cells also release cytokines to attract more immune cells, creating a positive feedback loop of cytokine creation. Massive accumulation of pro-inflammatory cytokines producing-immune cells in the lungs could increase the severity of COVID-19 patients.